Infrared gas burner



Dec- 5 1964 c. l.. Goss ETAL 3,151,227

INFRARED GAS BURNER Filed April 24, 1962 Pig. j

U1 L? INVENTOR;

United States Patet 3,15 1,22 7 Patented Dec. 1 5, l 964 This invention relates to infrared gas burners and in particular it comprises novel structure including a mixing chamber and a burner plate on the surface of which nearly incandescent combustion takes place.

Infrared gas burners are known in the art and are exemplified by such United States patents as Burns et al., Number 1,896,286, Schwank, Number 2,775,294l and Schwank et al., Number 2,870,830. 1t will be noted that in the prior artt much attention has been given to the provision of adequate mixing of the gas and air combustion mixture, and to avoiding the phenomenon termed flash- 'back in which combustion takes place below the burner plate thereby stopping normal operation and in many cases destroying the complete burner.

Despite the eiorts made in overcoming iiashback and the like, our studies have shown that this problem remains in the art to a degree. Moreover, there are other problems that provide limitations on the eifectiveness of these burners. For example, we have found that the available gas burners are characterized by a moderately limited temperature of operation. When temperatures of a higher level are experienced, fracture of the burner plates or separation from the supporting structure and destruction of the mixing chamber can occur.

It is, therefore, a primary object of the present invention to provide an infrared gas burner structure that can operate at a considerably higher temperature than gas burner structures presently available, that is characterized by thermal expansion characteristics such that its various parts fuse rather than fracture or separate from one another at excessive temperatures, that is easily prepared from inexpensive materials and with skills presently available in the art.

Another object is to provide a novel burner plate for a structure in accordance with the foregoing object.

These and other objects are attained in accordance with our discoveries by providing an infrared gas burner comprising a thin wall ceramic honeycomb coveror burner plate and a body portion defining a mixing chamber having a gas inlet and supporting structure for the cover plate. Each of the parts is composed of a low thermal expansion lithium aluminosilicate ceramic material and they are joined to one anotl rer with a foamed ceramic cement of low thermal expansion to result in an essentially unitary structure. With the parts having particular characteristics as indicated hereinafter, there results an infrared burner that can safely be operated at very high temperatures for extended periods, that will not fracture upon being repeatedly subjected to excessive temperatures or temperature changes, that is free from the propensity toward flashback and yet is economical and is easily produced.

Y The invention will be described in detail in conjunction with the attached drawings in which:

FIG. 1 is a top view, with part of the vcover plate broken away, of an infrared gas burner of the present invention;

FIG. 2 is a sectional view of the infrared gas burner of FIG. 1 taken along line II--IIg and FIG. 3 is a perspective view of a part of an infrared gas burner of the invention.

Referring now to the drawings, a Igas rburner 8 in accordance with this invention is composed of a body member it) having an integral or separately attached centrally disposed inlet tube l2 extending from one side. The top surface or cover plate of the gas burner 8 is a ceramic honeycomb structure and is indicated by the numeral 14. The cover member 14 is attached to the body it? by means of a particular foamed ceramic cement indicated by the numeral 16 in the drawings. r-l`he tube 12 is not essential to the invention and can be omitted when not desired.

As will be noted in FIGS, 1 and 2, the inlet tube 12, when used, suitably extends into the body 10 terminating near its Wall l@ that is opposite the wall 2d through which the tube enters. The zone 22. within body it? between the cover member 14 and the bottom wall 2o comprises a mixing chamber where air and the fuel, such as natural gas, propane, butane, manufactured gas, town gas, or the like, thoroughly mix prior to passing upwardly through the cover member M for combustion. While body mem- Iber l@ is shown as a generally dish-shaped structure, it should be understood that various other shapes and inlet tube orientations can be used Without departing from the scope of the invention.

Characterizing features of the invention include the use or a particular honeycomb structure `as, the cover member, and the manner of joining the cover member or yplate to the burner body. As is shown in the drawings, the -honeycomb cover member i4 is characterized by a large number of unobstructed gas paths 30 that extend from the bottom surface 32 or the cover member through to its top surface 34. These unobstructed gas paths are defined and separated from one another by thin ceramic walls Se. By unobstructed, we mean that no structure exists within the gas ilow paths that would prevent ow. The ceramic walls deining those iiow paths can .be arranged in triangular or polygonal shape `as desired.

he cover honeycomb it attached to the body member 1? at the upper portion of the side walls of that member. That is accomplished by use of a low expansion ceramic cement in that is foamed in place and joins the cover plate i4 and body member it) to a unitary structure.

In the use of a burner of the invention, a source of a gaseous fuel and air enter the burner body il@ through the inlet tube i2 or any other Suitable inlet means. The fuel and air can be provided by sources connected to a mixing head that is attached to the inlet means, or the pressure of the fuel gas can be used with suitable means to aspirate air into the stream. Other procedures, none of which form any part of the invention, can be used as Well. The gases enter the enlarged space 22. within the burner body itl and thoroughly mix therein. For mixing purposes, the inlet tube can be designed so that the gases impinge on a Wall of the body member 1d, or bailles (not shown) can be included within the body to eliect that object. The mixed gases then pass through the gas paths 3b in the cover plate to the upper surface where they are combusted, raising the surface of the plate to nearly incandescence. Any suitable conventional combust-ion initiator can be used as desired.

Ceramic honeycomb structures that are used for burner plates in accordance with the present invention have certain definite characteristics that are essential for proper operation and to provide the outstanding advantages characteristic of the invention. The honeycomb, aswell as the body member and the foaming cement, must be formed of ceramic materials sintered to a unitary structure and that provide a low coefficient of thermal expansion in the tired state. Ceramic materials that provide such a coeflcient of expansion inherentlyjhave a high heat conductivity on the order of at least 0.0020 cal./ sec. cm.

C. or higher. While a high heat conductivity generally would be considered disadvantageous,V that property is not deleterious in the present invention but is compensated by the high specific heat and the thinness of the walls defining the honeycomb. The Wall thickness is below a 0.01 inch and generally is in the range of 0.005 to 0.008

inch. Even thinner walls may be used if desired. AS

will be noted from the drawings, two `walls come together to point contact at various locations in the honeycomb.

At that point, the overall wall thickness may approach 0.02 inch. Such Yjunctures, however, involve but a few percent (eg, less than live percent) of the total and therefore are insignificant. The length of the gas flow passages, and therefore vthe thickness of the burner plate, must be at least 0.2 inch and preferably is at least 0.375 inch. Usually, the plate thickness does not exceed about three quarters of an inch.

The high specific heat of the ceramic of the honeycomb contributes to heat absorption whereby good infrared radiation is achieved, and is on the order of 0.20 calorie per gram lor higher at 25 C. The high thermal conductivity tends to conduct heat away from the burner surface. Normally that would contribute to flashback, but it does not do so in our invention. The gas passages have a small cross-sectional area and accordingly gas velocity, for a given gas inlet pressure, is relatively high. Therefore, the gas abstracts the heat absorbed by or conducted along the thinvwalls, keeping the upstream portion of those walls at a low temperature and avoiding the tendency to ashback. For example, with a gas flow path having a cross-sectional area of one square millimeter, an inlet gas pressure of 4 to 7 inches of water and a wall thickness of 0.005 to 0.010 inch, a glowing Zone results in the honeycomb burner plate that extends from its burning surface inwardly toV about 0.06 inch. The temperature in that zone is on the order of 1500" to 1700 F. Immediately adjacent the glowing Zone is a transition zone of about 0.01 to 0.03 inch in length over which the temperature varies from about 1500 F. at the edge of the glowing zone downto about 300 F. at the bottom or upstream limit of the transition zone. This gradient and the relatively cold upstream condition arebrought about by the effective cooling action of the high velocity fuel mixture, resulting from the small crosssectional area of the gas passage, flowing against the thin walls, the minimal mass of the walls permitting rapid heat removal.

The gas passages have an average port or cross-sectional area at the burning surface that does not exceed 0.006`

square inch and preferably is below about 0.005 square inch; The gas passages or ports are, of course, uniformly distributed throughout the burner plate and their crosssectional areas in the aggregate must provide at least'V 50 percent and preferably at least 60 percent of the total area of the burner plate surface. It has been found that these limitations are critical. For example-With a larger individual port area, the gas velocity for a constant gas input pressure would drop below the Value nessary to counteract or olfset the llame front velocity, i.e. the rate at which the lame will ignite fresh gas coming in contact with the llame. Furthermore, the lower gas velocity would not adequately cool the ceramic walls. Accordingly, a larger cross-sectional area for the ports would render the structure more subject to flashback.

When an open area less than 50 percent of the total surface area of the burner plate, for the standard cornniercial gas line pressures of about 4 to 7 inches of water, unduly high gas velocity through the gas paths would absorption would occur and the ceramic would Vact as a liame arrestor.

An important advantage of the requirement for the large open area for the burner plates is that, for a given gas inlet pressure, it provides a greaterr heat input in consequence of the .lessened pressure drop across the thickness ofthe burner plate. Accordingly, a greater Volumeof gas Vper unit of time'is passed through the burn-erplate and burned with a correspondingly greater production of heat. This advantage results in the ability to provide smaller and more compact burner units to meet a given heat output requirement since the open area provides a greater total gas throughput and therefore a greaterl total heat production.

The Very thin walls of theceramic honeycomb that are necessary in producing suitable burner plates for this inventioncannot be achieved by molding processes. To attain the strength in the green lstate necessary for a molding process would require far thicker Walls. The thin walls of the honeycomby are not fragile, contrary to expectation. The low expansion lithium aluminosilicates sinter to a surprisingly strong and dense structure. Accordingly, operation to higher temperatures without mechanical destruction is possible. And this is achieved despite the severe thermal gradient to which this seemingly fragile structure is subjected.

p Ceramic honeycomb bodies that serve as the burner plate in this invention and conform to the limitations stated can'be preparedl by several processes. For example, a pulverized ceramicmaterial can be admixed with a suitable binder andy then extruded to a ribbon form. The resulting ribbon can be further shaped, if desired, and assembled either by itself or with other ribbons of this material, to the desired honeycomb shape. The resulting assembly is then sintered to a unitary structure. Preferably, however, the ceramic honeycomb body is prepared by coating a suitable carrier with a mixture of a pulverized ceramic and a binder, crimping the resulting coated carrier and then assembling it'to the desired shape alone or with another coated carrier that need not be crimped. The assembled body is then heated to a temperature sufficient to sinter it to a unitary structure as more 'fully detailed hereinafter. This latter procedure is, generally the process set forth in the copending application of Robert Z. Hollenbach, f Serial Number 759,706, led September 8, 1958, now Patent The purpose of the binder is to bond the unred ceramic material to the carrier, to impart green strength to the coated carrier and to retain the formed uniired article in the desired shape after forming and prior to sintering. In order thatrthe resultant article be essentially all ceramic material havingV a low coefficient of thermal expansion7 it is preferred to use an organic bind er, especiallyV those that are heat curable or thermosetting that can be removed by decomposition and/or 'volatilization when the article is fired. Among `the many materials having the requisite, well known characteristics of binders that can be used in the process are such natural materials as gum arabic, colophony and shellac and such synthetic organic resins as acrylate resins, methacrylate resins, alkyd resins, cellulose derivatives, coumarone indene resins, phenolic resins', polyamides, polyesters, resorcinol resins, styrene resins, terpene resins, urea resins,

vinyl resins, epoxy resins, chlorinated parailins and melamine resins.

The purpose of the carrier is to provide support for the uniired coating to allow it to be formed to the desired shape prior to sintering the ceramic coating. Tea bag paper is a preferred carrier and a list of other suitable materials is disclosed in the aforementioned Hollenbach patent, to which reference can be made. Tea bag paper, as Well as other organic ilm materials, substantially decompose upon tiring and thus result in an article consisting almost entirely or" ceramic material.

In order to produce a structure having characteristics suitable for an infrared burner plate in accordance with our discoveries, it is essential that ceramic materials be used that have a low coelcient of thermal expansion in the tired state on the order of about minus l0 to plus 2t) times l()'I C. over an extended temperature range. Suitable ceramic materials for the burner plate include lithium aluminosilicates such, for example, as glass or crystalline petalite and beta spodumene, glass-ceramics having a lithium aluminosilicate base and especially those made in accordance with Example l of United States patent to Stookey, Number 2,920,971, as well as mixtures of any of the foregoing materials. Petalite glass-ceramic mixtures generally include about l0 to 40 weight percent of the glass-ceramic and the remainder petalite. Beta spodumene-petalite mixtures usually contain about l te 4 parts of petalite for each 4 to l parts of beta spodumene. Those materials normally are used in a particle size of about minus 2U() mesh (Tyler) or finer, depending on the wall thickness desired in the resulting article.

Structures are assembled from ceramic coated carriers in a variety of ways, and the resulting structures are called honeycombs, a term which in this specification means a unitary body having a multitude of unobstructed gas paths of predetermined size and shape, each such gas path being dened by thin ceramic Walls and extending between and terminating in vopposed surfaces. These structures can be assembled from multiple layers of lm corrugated with the same pattern with alternate layers laterally disposed a distance equal to half of the width of the individual pattern so that layers do not nest with each other. The honeycomb structure can also be formed from rolling alternate layers of crimped and uncrimped coated carriers until the desired shape is formed. The structure can also be formed by assembling to a stack alternate crimped and uncrimped coated carriers until the desired dimensions are attained. Since the minimum thickness for suitable burner plates (i.e. length of gas ilow paths) is at least 0.2 inch, the structure is either formed in that size or a fired body is cut to the desired thickness. Other ways of assembling these honeycombs will be apparent to those skilled in the art.

The tiring of the green structure er matrix, however formed, is accomplished in the normal manner for ceramic tiring by placing the article in a furnace and heating it at at rate slow enough to prevent breakage up to a temperature high enough to cause the ceramic particles to sinter. While the tiring schedule, including heating rates and sintering temperatures, will vary depending upon the ceramic material utilized, the size and shape of the article formed, and the atmosphere used, the details of such schedules are not critical and suitable conditions are readily determinable by one skilled in the art of tiring ceramic articles.

The burner body member lil and inlet tube l2 for use in the invention can be made by any of the techniques available in the ceramic and glass arts. For example, they can be cast from suitable ceramic mixtures, as by making a slip of the ceramic materials, casting the slip, and ythen firing to a unitary structure. They can also be formed of the suitable glass, ceramics by the conventional hot glass forming techniques well known in the art for these materials. lr the inlet tube is separately formed, it can readily be joined to the body member lll by use o the foaming cements referred to hereinafter. As already noted, the burner body member must have a low coeliicient of thermal expansion. Accordingly, it is made from one or more of the materials mentioned above as suitable in forming the honeycomb burner plate.

The heater body and honeycomb cover plate are joined to provide a unitary structure by use of a ceramic cement that will foam and readily bond those members and that has a low coeflicient of thermal expansion in the foarned state. Cement for this purpose has a composition, by

eight, of l to l6 percent ot" lead oxide, l to l5 percent of a llux, l to 6 percent of silicon carbide, l to 6 percent of S03 and substantially all or" the remainder, and at least about 70 percent of the total cement composition, a lithium aluminosilicate ceramic such as glass or crystalline petalite and beta spodurnene, a glass-ceramic having a lithium aluminosilicate base such as that of Example I in the mentioned Stookey patent, as well as mixtures of any ot the foregoing. Glass petalite is the preferred ceramic. rypical ilux materials include the iluorides and oxides of magnesium, calcium, strontium, barium, zinc, cadmium, lead, lithium, sodium and potassium. Suitably a mixture of oxide and iluoride fluxes is used. The S03 content of the batch is provided by, for example, a compound such as calcium sulfate, barium sulfate, strontium sulfate, magnesium sulfate, Zinc sulfate, cadmium sulfate, lead sulfate, sodium sulfate, potassium sulfate or lithium sulfate. t will be apparent that the use of any of these compounds provides both the S03 and an oxide ilux. Lead sulfate, which also may be used, provides the essential lead oxide and S03. into the spaces between the parts to be joined and then firing the unit to a temperature of about 1050 to l`150 C. or more until foaming and sintering are complete. Thereafter, the unit is cooled to handling temperature.

The invention will be described further in conjunction with the following example in which the details are given by way ci illustration and not by way of limitation.

ln this example, la ceramic composition is made of 75 parts by Weight of petalite and 25 parts by weight of a glass-cenamic having the following approximate composition, by oxide analysis, in Weight per cent: 70 percent SiGg, 18 percent Al203, 5 percent T102, 3 percent Li20, 3 percent Mg() and l percent ZnO. The composition is ball-milled to a minus 200 mesh (Tyler) particle size. A solution of the following composition is added to 2160 grams `of the ceramic material in the ball mill:

64() cc. of isopropanol 860 cc. of ethyl-acetate 180 cc ot Versamid 115 48() cc. Hysol 611 l Versamid ll5 is the trade name ott a thermoplastic polymer supplied by General Mills, Inc. It is prepared by condensation of polymerized unsaturated fatty acids, such as dilinoleic acid, with aliphatic amines such as ehtylene diamine. Hysol 6lll is the trade name of an epoxy resin solution, supplied by Houghton Laboratories, Inc. containing 5'7 percent by weight of epoxy resin having a viscosity of about 2.5-4.0 poise at 25 C, an epoxide equivalent (gr-ams of resin containing l g, chemical equivalent of epoxy) of 595150, and a melting range of 73-35C.

The ceramic material and the binder are further ballmilled for about three hours to produce a uniform suspension. A porous natural cellulose paper', commonly known as 3%. pound tea brag paper, cut to a width of 4 inches is then dipped into the suspension .and dried by heating to 129 C. for 2 minutes. The dried, coated paper is then heated to 180 C. and crimped to produce a pattern, taken in cross-section, in the shape of an isosceles triangle with legs about 0.07 inch long and an open base about 0.1 inch wide. The crimped, uniired, coated paper is rolled up simultaneously with a sheet of tea bag paper of the same width, which has been coated in the The cement is used by pouring it same manner but not crimped, upon a 2-inch diameter reel until an annular cylinder with an outside diameter of about 23 inches is obtained. Preferably, the uncrimped coated paper is not dried prior to the roll-up operation, but this paper is dried by forcing air heated to about 120 C. through the channels of the annular `cylinder as they are formed during the roll-up operation.

The unlired matrix body is then placed in a'furnace chamber and heated in accordance with the following schedule: i

Temperature Range: Firing Rate Thereafter the matrix yis cooled to handling temperature. A body member as shown in the Vdrawing is made from a ceramic composition as stated above. The ball-milled ceramic mixture is tempered with water to a slip consistency, and cast in an absorbent mold. After the cast article has developed strength by partial drying, the mold is removed and the body member is thoroughly dried by heating at 125 C. for 10 hours. Thereafter, it is tired at 1250 C. for 8 hours. f

The body and a section of honeycomb, i.e. about @Vs inch thick section sawed from the matrix, are assembled. Then a cement having the following composition, by

weight, is used to join these members: 8.72 percent of zinc oxide, 1.3 percent of calcium fluoride, 3.46 percent of silicon carbide, 1.93 percent of S03, 6.81 perce-nt of PbO and the remainder glass petalite. cement composition is dispersed in a mixture containing 75 weight percent of butyl alcohol and 25 weight percent of toluene and is wet ball-milled to thoroughly mix the batch. This cement is poured in the annular spaces in the assembly. The resulting assembly is then placed in a furnace and raised to 100 C. yat a rate of 2 C./min. After 2 hours at 100 C., the temperature is raised at 5 vCJminute to 11.50 C. and is held at 1150 C. for one hour and 15 minutesto permit the foaming action to be completed. It is then furnace coo-red ata rate of 5 C./rnin. to handling temperature. Y

Ceramic infrared gas burners prepared as just described have been actually tested and have been found to be entirely satisfactory. `Extended operation with the burning surface cycled at temperatures between room temperature and about 1100 C. did not adversely affect the structure, neither causing fracture nor separation of the cover from the body fof the burner. As far as is known, no other ceramic burner for this purpose could successfully operate by cycling at temperatures between room tempera-ture and about 1100" C. for any material per-iod of time without destruction occurring.

From the discussion and description, it is evident that the present invention constitutes an important advance in the art of infrared gas burners. By this invention such burners are provided that can oper-ate at higher temperatures for longer periods of time than other burners, yet the improved burners are more durable and safe to operate and are relatively inexpensive to produce. These outstanding advantages are largely the result of using the lithium aluminosilicate ceramics of low thermal coefficient of expansion, the provision of a truly unitary stmcture by reactively joining the cover and body member with the. cement foamed'in situ, and the formation of la dense, strong honeycomb with a resulting high ther A batch of this.

S we have explained the principle of our invention and have illustrated and described what we now believe to ,represent its best embodiment. However, it should be understood that, within the scope of the appended claims, the

invention can be practiced otherwise than as specifically described.

l; An infrared gas burner comprising la ceramic burner body member defining a mixing zone, inlet means through la wall of the body member for the introduction Vof air and fuel, a Vceramic cover plate on said burner body member, and a foamed ceramic cement joining said lcover plate `and body member to one another providing a unitary structure, said cover plate comprising a thin walled ceramic honeycomb yhaving a pair of opposed surfaces and a plurality of unobstructed gas passages extending between and terminating in those surfaces, the body member, cover plate and foamed cement yall having a low and substantially similar coefliclient of thermal expansion; v

2. A gas burner according lto claim 1 in which each gas passage has a cross-sectional area Vthat is less than 0.006 square inch and the collected cross-sectional areas of the gas passages provide an 4open space of at least 50 percent of said surfaces.V

3. A-gas burner according to claim 2 in whichv the ceramic of the body member, the honeycomb and the cementcomprises a lithium aluminosi-licate'. f.

4. A gas burner according tok claim 2, in which the ceramic of the 'body member and the honeycomb comprises a lithium aluminosilicate and ,thecomposition of the cement is, by weight, 1 to 16 percent of lead oxide,

1 to 15 percent of a flux, 1 to 6 percent of silicon carbon, f

, l` to 6 percent olf S03 and the remainder a lithium aluminosilicate ceramic, said lithium aluminosilicate ceramic comprising at least 70 percent of the total cement composition.

5. An infrared gas burner comprising a ceramic burner body member defining a mixing zone, inlet means througha wall of the body member for the introduction of air and fuel, a ceramic cover plate on the burner body member, and a foarned ceramic cement joining the cover plate and body member toene another providing a unitary structure, the cover plate comprising a thin walled honeycomb having an .inlet surface and an opposed combustion surface and a plurality of unobstructed gas passages uniformlydistributed throughouty the honeycomb and extending between and terminating in those surfaces, each gas passage havinga Vcross-sectional area that is less than 0.006 square inch, the collected cross-'sectional areas of the gas passages providing an open space Iat the Acombustion surface that is at least 50 percent of the total area of theV combustionr surface, the thickness of the walls of the honeycomb being below about 0.01 inch, the thickness of the honeycomb. being at least 0.2 inch, the body member, cover plate and formed cement all having a low and substantially similar coefficient of thermal expansion.

6. An infrared gas burner according toy claim 5, the cross-sectional areas of the gas passages and the thickness of walls of the honeycomb being such that.y the free space at the combustion surface is at least 60fpercent of the total area of that surface, andthe thickness of the honeycomb being at least 0.375 inch.

7.An infrared gas burner according to claim 6 in which the ceramic of the body'member, the honeycomb and the cement comprises a lithium 'aluminosilicate 8. An infrared gas burner according to claim 6 in which the ceramic of the body member'and of the'honeycomb comprises a lithium aluminosilicate and the compositionof the cement is, by weight, 1 to 16 percent of lead oxide, 1 to l5 percent of a flux, 1 to 6 percent of silicon carbide, 1 to 6 percent of S03 and the remainder a `lithium,aluminosilicate ceramic, said lithiumv alumino- Q i@ silicate ceramic comprising at least 70 pern of the 1,939,476 Werner Dec. 12, 1933 total cement composition. 2,920,971 Smokey Jan. 12, 1960 3,041,199 Griffith et :211 June 26, 1962 References Cited in the le of this patent UNITED STATES PATENTS 5 FOREIGN PATENTS 1,222,922 Bone et a1. Apr, 17, 1917 5261559 Canada Aug 29 1961 

1. AN INFRARED GAS BURNER COMPRISING A CERAMIC BURNER BODY MEMBER DEFINING A MIXING ZONE, INLET MEANS THROUGH A WALL OF THE BODY MEMBER FOR THE INTRODUCTION OF AIR AND FUEL, A CERAMIC COVER PLATE ON SAID BURNER BODY MEMBER, AND A FOAMED-CERAMIC CEMENT JOINING SAID COVER PLATE AND BODY MEMBER TO ONE ANOTHER PROVIDING A UNITARY STRUCTURE, SAID COVER PLATE COMPRISING A THIN WALLED CERAMIC HONEYCOMB HAVING A PAIR OF OPPOSED SURFACES AND A PLURALITY OF UNOBSTRUCTED GAS PASSAGES EXTENDING BETWEEN AND TERMINATING IN THOSE SURFACES, THE BODY MEMBER, COVER PLATE AND FORMED CEMENT ALL HAVING A LOW AND SUBSTANTIALLY SIMILAR COEFFICIENT OF THERMAL EXPANSION. 